Verify the carb is getting plenty of fuel, running lean will cause this.
Verify the timing is close to spec's, it could idle fine and still be off to far to make power.

How does it respond ? buck around, backfire, list all the symptoms, how does it start up once it dies?
Whats the history of carburetor? fuel pump? fuel lines? tank?
What color are the sparkplug tips?

Joe